NATO Secretary-General Jens Stoltenberg has said Afghan security forces were dying in increasing numbers because they have assumed a leading role in the fight against the Taliban.

Speaking during a visit to Afghanistan’s eastern city of Herat on November 7, Stoltenberg said Afghan forces "have already been in the lead and have had the main responsibility already for almost a year.”

Figures this week showed that the number of Afghan soldiers and police officers killed in battle reached 4,634 so far this year -- a 6.5 percent rise compared to 2013.

The second-ranking U.S. commander in Afghanistan, Lieutenant General Joseph Anderson, said on November 5 that "those numbers are not sustainable in the long-term."

NATO is to conclude its combat mission in Afghanistan by the end of 2014. Some 12,000 U.S. and NATO troops will remain in a training and support capacity.